1. Field of the Invention
The present invention relates to tube shields for use in power and recovery boilers. More specifically, the invention relates to a method and apparatus for attaching tube shields to the tubes of a power or recovery boiler which will prevent rotation and axial slippage of the tube shields with respect to the tubes.
2. Related Art
Soot blowers are high-pressure steam blowers used to remove excessive soot and ash which form in the furnace portion of a boiler. As shown in FIGS. 7 and 8, the boiler tube assemblies 100 used in power and recovery boilers commonly incorporate passageways to accommodate travel of a soot-blower lance 200 for the cleaning of soot and ash from the boiler. Tube shields 110 are installed on those tubes 120 which are affected by soot blower cleaning and high pressure boiler turbulence.
Presently, there are three common systems in use for attaching tube shields to the boiler tubes, as illustrated in FIGS. 1-6. The first of these systems is the U-bolt system, shown in FIGS. 1 and 4. In the U-bolt system, the tube shields 110a are formed to cover approximately half of the circumference of the tubes 120. For each tube shield, the closed portion of two U-bolts 130 is placed around the uncovered portion of tube 120 adjacent the ends of the tube shield 110a, and two flat bars 132 with holes at each end are placed over the threaded ends of U-bolts 130. Nuts 134 are then screwed onto the threaded ends of U-bolts 130, securing the tube shield 110a to tube 120.
Once tube shields 110a have been installed using the U-bolt system, and the boiler is placed in service, the temperature fluctuations inside the boiler cause the tube assemblies to expand and contract, resulting in loosening of the nuts. As a result, tube shields 110a and U-bolts 130 tend to rotate or slip axially with respect to tubes 120.
The second system is the snap system, illustrated in FIGS. 2 and 5. In this system, the tube shields 110b are formed to cover approximately two-thirds of the circumference of the tubes 120, and are bent to have a slightly smaller radius than tubes 120, allowing them to be snapped onto tubes 120. As with the U-bolt system, once tube shields 110b have been installed and the boiler is placed in service, the temperature fluctuations inside the boiler cause the tube assemblies to expand and contract, resulting in loosening of snapped-on tube shields 110b, rotation and axial slippage.
The third system is the welded U-strap system, illustrated in FIGS. 3 and 6. In this system, the tube shields 110c are formed to cover approximately half of the circumference of the tubes 120. One-inch wide strips 140 are rolled around the uncovered portion of tube 120 adjacent the ends of tube shield 110c, are lapped over the edges of tube shield 110c, and are welded in place. Once tube shields 110c have been installed, and the boiler has been placed in service, the high temperature in the boiler and tubes 120 causes strips 140 to expand with tubes 120. However, when tubes 120 contract, straps 140 do not, again resulting in loosening of tube shields 110c, rotation, and axial slippage.
A fourth system is disclosed in U.S. Pat. No. 4,619,314 to Shimoda. In this system, a tube is provided with a semi-cylindrical protector which is formed with pairs of projections 4 and 5 which extend beyond the tube and which receive pins 8 and 9. Studs 13 are welded to the tube adjacent the inner surfaces of projections 4 and 5 to prevent rotation and axial displacement of the protector with respect to the tube. Although this system overcomes the rotation and slippage problems of the preceding three systems, it requires the provision of numerous pins for each protector.
It is the solution of these problems to which the present invention is directed.